Some wastewater treatment facilities, particularly those that have limited space, employ stacked tank clarifiers instead of side by side tank clarifiers. In a stacked tank clarifier, one tank is located above a second tank in vertically stacked relation. In such stacked tank clarifiers, it is necessary to clean matter (sludge) from the ceiling of the lower tank as well as from the floor of the lower tank. This is because some matter in the lower tank floats up against the ceiling of the lower tank and remains there due to minimal waterflow in the lower tank.
A sludge collector mechanism is provided in the upper tank, and a sludge collector mechanism is also provided in the lower tank. The sludge collector mechanism in the upper tank includes transverse flights which are rigidly carried on a pair of spaced apart drive chains in a circuit along the floor of the upper tank as well as along the surface of the water in the upper tank so as to scrape the floor of the upper tank and skim the surface of the water in the upper tank.
In such prior art stacked tanks, the sludge collector mechanism in the lower tank typically includes rigid transverse flights which are carried on a pair of spaced apart drive chains in a circuit along the floor of the lower tank to scrape the floor of the lower tank, and along the ceiling of the lower tank. These flights in the lower tank do not effectively scrape the ceiling of the lower tank. The rigid flights do not effectively scrape the ceiling of the lower tank because the surface of the ceiling is not uniform and because the rigid flights become worn to conform to the lowest surfaces of the ceiling of the lower tank, and become ineffective for scraping the other surfaces of the ceiling of the lower tank. There may be as much as 1" of tolerance in the surface of the ceiling of the lower tank; i.e., the lowest surfaces of the ceiling of the lower tank may be as much as 1" lower than the uppermost surfaces of the ceiling of the lower tank.
The flights used to scrape the floor of the lower tank must be rigid because the sludge that collects at the floor of the lower tank is substantially heavier than the sludge that collects at the ceiling of the lower tank, and a rigid collector flight is required to move the sludge along the floor of the tank.
One prior art structure for scraping the ceiling of a lower tank of stacked settling tanks is disclosed in Hitachi Japanese Patent Disclosure 58-43126. The apparatus disclosed in Japanese Patent Disclosure 58-43126, includes wooden flights pivotally attached to a chain.
Other prior art apparatus for scraping the ceiling of a lower tank include flexible members rigidly attached to the rigid flights carried by the pair of drive chains. A problem with such apparatus is that the flexible members contact the walls and floor of the lower tank in addition to the ceiling of the lower tank, and tend to wear quickly. Another problem with such apparatus is that the flexible members tend to get hung up at the transitions between the walls of the tank and the floor of the tank, thereby interfering with proper movement of the chains in the lower tank, and resulting in chain and sprocket wear. Another deficiency of such apparatus is that the flexible members interfere with proper scraping of the floor of the lower tank by raising the rigid flights off of the floor. Another problem with such apparatus is that the flexible members quickly become worn to conform to the lowest surfaces of the ceiling of the lower tank, and become ineffective for scraping remaining surfaces of the ceiling of the lower tank.